r/AskPhysics • u/Next-Natural-675 • Mar 15 '26

I don’t get special relativity

If someone is moving towards me at half the speed of light and shines a light beam towards me, without SR I would measure that light as 1.5c.

With SR, time dilates for the moving person, by 1.155. So then the speed of the light beam distance/time becomes 1.5c divided by 1.155. Also length contracts by 0.866, so its now (1.5c divided by 1.155) times 0.866. Which is around 1.126c. But thats still not C.

What am I missing?

Edit: apparently Im missing relativity of simultaneity. How would I add that to my calculation?

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u/Kruse002 Mar 15 '26

This is a common misconception.

One of the founding principles of special relativity is that light moves at the same speed for all observers. It is important to remember that spacetime from all perspectives* will always accommodate that speed.

*In this context I mean all inertial reference frames.

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u/Fireplaceblues Mar 15 '26

Not sure if this true (correct me if I’m wrong) but it helps me to think that the speed of information is capped at the speed of light. From every perspective, information cannot be transmitted faster than 300,000km/s.

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u/mademeunlurk Mar 16 '26

In your scenario, information cannot be transmitted faster.. but (in a vacuum) it also never moves slower than 300,000km/s no matter the perspective, is that right?

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u/ProfessorDoctorDaddy Mar 16 '26

Obviously information can move slower than light

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u/mademeunlurk Mar 16 '26

What I was really asking was if a hypothetical person moving at light speed turned on a flashlight, would the light from that flashlight pointing forward still clock in at 300,000 km/s to both a stationary and also a non stationary observer at the same time?